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Plants and Equipment                                                                                341


                    I don’t care for centrifugal feed pumps in heating  occurrence with a heavy load, the turbine pump output
               plants because they can’t handle the pressure variations.  will only increase a little bit but the centrifugal pump
               Take the typical heating boiler plant. Both centrifugal  will increase its delivery over twice as much. That ad-
               and turbine pumps can be obtained to produce a design  ditional water consumes more of the boiler’s heat input
               flow of about 31 gpm (15,500 pph) at the normal boiler  leaving less to make steam so the pressure drops further.
               operating pressure of 12 psig (31.7 feet). The density of  I think this shows that a poor choice in boiler feed pump
               water for this example is assumed to be 54.55 pounds  selection on low pressure boilers can produce serious
               per cubic feet, 175°F water, which means the head rela-  headaches for the boiler operator. Replacing those cen-
               tionship is 2.64 feet per psi. There is a big difference in  trifugals with turbine pumps can reduce the swinging
               their operation as the pressure changes. They’re selected  pressure  problems  encountered  in  some  plants  and
               for when the boiler runs up to the limit of the safety  eliminate others because the pumps create the problem.
               valves (15 psig or 39.5 feet).
                    My concern with using centrifugal pumps is that  Screw and Gear Pumps
               any external pressure effects can result in total loss of     Screw and gear pumps are used principally for
               water delivery (Figure 10-80). The curve as shown will  fuel and lubricating oils. They can be more efficient at
               deliver water to the boiler but changes in such things  moving liquids with viscosities higher than water than
               as level in the deaerator or feedwater tank can prevent  other types of pumps and are capable of producing high
               delivery. The values of head used on this curve assume  differential pressures in a small package. Since they’re
               that the pressure drop through the piping is negligible  positive displacement pumps one running at 3500 rpm
               and the level in the boiler is the same as the level in the  can be half the size of one running at 1750 rpm to pump
               feedwater tank. From this curve it’s apparent that a drop  the same amount of oil.
               in level at the feedwater tank of a couple of feet will in-     Screw and gear pumps are positive displacement
               crease the head requirement for the pump to the point  pumps and work pretty much alike. The pumps use two
               that the centrifugal can’t deliver any water until the  or more machined rotors that mesh closely together and
               boiler water level or pressure drops enough to produce  produce a moving cavity as they rotate with each other.
               a differential the pump can overcome.                The cavity opens at the suction end and is sealed as the
                    On the other hand, any drop in boiler pressure will  rotors turn then the cavity travels to the discharge end
               be accelerated by a centrifugal pump with a relatively  of the pump to deliver the liquid at the discharge pres-
               flat curve. If the boiler pressure drops to 8 psi, a typical  sure. The liquid serves to lubricate the rotors to prevent
                                                                    them rubbing each other or the pump casing. The ends
                                                                    of the rotors are enlarged to increase bearing surfaces to
                                                                    balance the axial forces or shaft bearings take the thrust.
                                                                        Some liquid is squeezed between rotors and cas-
                                                                    ing in the opposite direction of the moving cavity, the
                                                                    amount depending on pump construction and wear.
                                                                    Smaller pumps usually have one rotor or gear that is
                                                                    driven and the rest of the rotating parts are driven by it
                                                                    in turn. Larger pumps and pumps that produce high dif-
                                                                    ferentials or pump very low viscosity liquids can have
                                                                    external gearing so each rotating element is driven. That
                                                                    reduces the amount of force that has to be transferred
                                                                    through the thin film of liquid between rotating parts,
                                                                    replacing it with the lubrication of the external gears.
                                                                        The quality of internal lubrication is dependent
                                                                    on differential pressure and pump speed. If the liquid
                                                                    is very viscous it will maintain a stronger liquid film
                                                                    between metal parts to prevent them rubbing. As the
                                                                    viscosity decreases the film gets thinner and will break
                                                                    to allow the metal parts to touch. The fluid bleeding
               Figure 10-80. Centrifugal and turbine pumps on low  back through the spaces between the metal parts is what
               pressure boilers                                     provides lubrication. The differential pressure between
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